This invention relates to liquid crystal displays (LCD's) suitable for use in radio telephones.
Liquid crystal displays used in radio telephones are frequently located on dedicated LCD modules separate from the main printed circuit board (PCB) of the telephone. There exist a number of advantages in having this kind of arrangement. The first advantage in most cases is to save space. Main PCB's used in radio telephones are often packed with electrical components so densely that there is little room left for the relatively large components such as the display. By providing the LCD display on a dedicated module which is located above the main PCB the overall area of the main PCB may be reduced.
Another reason for including the LCD module separately from the main PCB is to permit a variety of mechanical layouts to be realised. If the LCD display needs to be raised from the main PCB in order for it to be closer to the front cover or if the LCD display needs to be angled in any way then a separate module provides for such arrangements in a more flexible way.
Perhaps the most important advantage of providing the LCD display on a separate module is to enable simpler assembly of the radio telephone. The advanced LCD modules which are becoming increasingly popular in radio telephones generally have low tolerances and do not integrate well in the standard production line techniques used to produce the main PCB's in radio telephones. For example, heat bonding techniques used to provide electrical connections to an LCD display would damage standard electronic components on a PCB. Therefore, to increase the overall rate of production of the radio telephone it is advantageous to produce the main PCB on one production line and to produce separate dedicated LCD modules on another production line. In this way the main PCB and the LCD module can be assembled together more simply towards the end of the production cycle thus saving in the overall time of production.
In one type of known radio telephone the liquid crystal display (LCD) module comprises a plastic frame designed to be clipped onto the main PCB of the radio telephone. The main PCB may be designed to include locating holes and suitable mounting locations for the LCD module. The LCD module may also comprise an LCD display and an LCD driver PCB, both of which may be mounted on the plastic frame by adhesive tape. The LCD driver PCB may be connected to the main PCB using a standard ribbon cable which plugs into a ribbon cable socket on the main PCB. It is also possible to include key contacts on the LCD module connected via another ribbon cable to the LCD driver PCB. These key contacts when operated by suitable keys act as side keys for the telephone in addition to keys provided by the main keypad. An alternative way of providing side keys for a radio telephone could be to attach a side key module to the main PCB of the radio telephone.
In practice these LCD modules tend to be problematic and do not provide adequately all the advantages referred to. Known LCD modules tend to impede the automated production cycle of a radio telephone by requiring labour intensive installation onto the main PCB. At the assembly stage on the main production line the ribbon cable of the LCD driver PCB must be placed manually into the ribbon cable connector of the main PCB. Likewise the addition of the optional side keys also increases the number of steps of the assembly stage on the main production line.
In known LCD modules the key contacts are positioned on the side of the support frame of the LCD module before the keys which operate them are introduced. When the keys are finally assembled in place above the key contacts there is an additional step in the production line to ensure the keys and the key contacts are aligned correctly. As the keys are usually added at the final stage of production when the cover of the telephone is fixed in place, the risk of misalignment of the keys and the key contacts is high. At the post production stage there still exists a problem of reliability. The keys are often placed loosely over the key contacts or are mounted on the cover of the telephone. As such the keys and the key contacts can easily misalign through normal use of the side keys resulting in their faulty operation.